AB1538 DETERMINATION OF ADENOSINE DEAMINASE (ADA) IN SYNOVIAL FLUID TO DIAGNOSE INFLAMMATORY ARTHROPATHIES
Authors: J. A. Bernal, J. C. Cortés-Quiroz, J. Ena, C. Raya-Santos, R. Gallego-Campuzano, A. Pons-Bas, L. Pons-Canet, M. Shalabi, J. M. Senabre-Gallego, G. Santos Soler, J. Rosas
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Adenosine Deaminase (ADA) determination is used to diagnose tuberculosis in several biological fluids, including synovial fluid (SF). Less known is the usefulness of ADA level in SF diagnosing other inflammatory arthropathies, and only a few reports could be found in the bibliography. In these reports ADA in SF seems higher in patients with septic arthritis, rheumatoid arthritis or crystal arthropathies compared with osteoarthritis. <h3>Objectives</h3> The objective is to evaluate the performance of ADA in SF in patients with joint effusion to diagnose inflammatory arthropathies. <h3>Methods</h3> All SF analyzed in 2022 at one center have been retrospectively reviewed. SF data have been collected (ADA levels and white blood cell) and definitive diagnosis of the treating physician -with all clinical and laboratory data- was recorded. Exclusion criteria were: bursas, prosthesis and hemarthros. Regarding the statistical analysis for the comparison of means of ADA in the different groups, the T-student test was used. <h3>Results</h3> There were 170 SF registered, 17 were excluded, thus finally 153 SF were included in the analysis. They were classified according to the diagnosis of the treating physician -the gold standard-: 95 inflammatory diseases (15 Spondyloarthritis, 9 Rheumatoid arthritis, 21 Gout, 10 Calcium Pyrophosphate arthropathy, 25 nonspecific arthritis, 2 Polymyalgia Rheumatica, 13 septic arthritis) and 58 non-inflammatory conditions (44 osteoarthritis, 9 meniscopathies, 2 osteonecrosis, 2 traumatic, 1 Baker cyst). ADA in SF was higher in inflammatory diseases -including septic arthritis- (mean 40.37 IU/L, SD 55.89) compared with non-inflammatory conditions (19.36, SD 12.20), and it was significant (p&lt;0.05). If it is compared with inflammatory arthropathies without septic arthritis (35.28, SD 77.55) it is also statistically significant (p&lt;0.05). Moreover, septic arthritis (72.43, SD 5.78) had higher levels of ADA in SF compared with other inflammatory arthritis (35.28, SD 63.66) (p&lt;0.05). Furthermore, ADA in SF was higher when SF white blood cell count (WBC) was over 2000/microL – usually considered inflammatory SF- (n=104, ADA mean 40.26, SD 54.28) compared with SF WBC &lt;2000/microL – usually considered non-inflammatory SF- (n=49, 15.72, SD 12.62) (p&lt;0.05). In our center when the ADA level in SF is over 30 IU/L is considered positive. Using as gold standard the physician diagnosis, the following results were calculated to diagnose inflammatory arthropathies in patients with synovial effusion: sensitivity (Sn) 40% (30.7-50.1), Specificity (Sp) 89.7%, (79.2-95.2), positive predictive value (PPV) 86.4% (73.3-93.6) and negative predictive value 47.7% (38.6-57). And it was calculated for septic arthritis in patients with inflammatory arthropathies: Sn 69.2% (42.4-87.3), Sp 64.6% (53.8-74.1), PPV 23.75% (13-39.2%) and NPV 93% (83.3-97.2). <h3>Conclusion</h3> ADA in SF is higher in inflammatory arthritis -including septic or not- than non-inflammatory arthropathies. Even more, in septic arthritis SF ADA is higher than in other inflammatory arthropathies. However, comparing with the gold standard ADA test has no good results except for Sp (89.7%) and PPV (86.4%) to diagnose inflammatory arthritis and NPV (93%) to diagnose septic arthritis. The results are limited given the low number of septic arthritis (13) and the sample will be expanded to recalculate and establish a more appropriate ADA threshold for diagnosing septic arthritis and inflammatory arthropathies. <h3>Acknowledgements</h3> The study was supported by a research grant from the Marina Baixa Association for Research in Rheumatology (AIRE-MB). <h3>Disclosure of Interests</h3> None Declared.